Protic ionic liquids (PILs) are being intensively studied as advanced materials for numerous specialized applications. Some of these applications include their use as proton transfer electrolytes and membranes in proton-exchange membrane fuel cells (PEMFCs), as well as their use as separation materials and solvent systems in chemical reactions.
A significant problem encountered with protic ionic liquids has been their propensity to degrade over time, and particularly, under elevated temperature conditions (e.g., temperatures greater than 100° C., as generally encountered in PEMFCs). The degradation is generally a result of retro-proton transfer between the constituent ions (i.e., conjugate base A− and conjugate acid BH+) of the protic ionic liquid, depicted as follows:(A−)(BH+)→HA+B
The resulting acid (HA) and base (B) lack strong Coulombic interactions, and thus, their generation results in a marked vaporization (i.e., volatility), and hence, increase in vapor pressure, of the ionic liquid. The degradation also leads, necessarily, to electrolyte loss. Particularly in the case of fuel cells, the electrolyte loss at higher temperatures is detrimental for several reasons, particularly in causing a reduced fuel cell operational lifetime and performance.
Accordingly, there is a need for new protic ionic liquid compositions that possess an improved stability, particularly at elevated temperatures. There is a particular need for such protic ionic liquids that also can be used as conductive components of proton exchange membranes.